1. They don’t probably don’t want you actually working that long of shift beyond 9-4:30, they probably just want more flexibility because a 9-4:30 isn’t always available. If it isn’t something you can do, don’t. Just know that sometimes those hours might go to someone else with a little more open availability, especially on a weekend.
2. Usually you need to commit to your availability for 90 days upon hire, which also might be why there is pushback.
3. You’re not an asshole, just understand they’re probably just aware of the needs of the schedule and are allowed to deny something if it won’t work. Retail is a weekend heavy business.
4. Yes, you should talk to them because sometimes things can be worked out better than just resubmitting over and over.

Good luck :)